Add support to `harper-cli lint` and `just lint` to check web pages

Some websites are goldmines of real-world English mistakes: Hacker News, Reddit, Medium.

It would be great if `harper-cli lint` could accept a URL (or multiple since it works with multiple inputs already)

I don't think Harper yet works with HTML - maybe I'm wrong?
Whether we do or not, it might still work better to first convert to plain text or to Markdown.
It would probably also be best either way to trim all the boilerplate from pages first and leave only the main content. Ages ago there was a tool called "Boilerpipe" in Java for this. I'm sure there are much better newer tools these days. LLMs probably use them so there's probably more to choose from than ever.

(This is probably a good issue to work on with the aid of an AI coding agent. If you pursue this, please use our official PR template and pay attention to our AI coding disclosure rules etc.)

Start by locating the harper-cli lint and just lint entry points, then inspect how multiple inputs are currently handled. Define how URL inputs, HTML or Markdown conversion, and boilerplate removal should work; done means the commands can lint one or more web pages while preserving existing input behavior.

Written by the indexing model from the issue text.